First off, ignore cable techs. They may want to help, but I've heard some pretty bizarre things from them. Realize they are not especially inclined for those sort of work. They'll take anybody with no background in cable or electronics and give them a short training course. There are definitely some that know their stuff but a lot of embellishment comes out of the others' mouths.
Never do I replace a modem, unless one of 1.5 things happens:
1) My ISP upgrades me to a higher tier for free. I don't have the cheapest tier but also don't feel like paying more per month, unless it's a long term investment like a higher DOCSIS standard and # of channels. I'd buy a new modem to get at least 25% higher speed.
1.5) Modem has failed from bad capacitors and is suspected to soon fall into thing #1 above.
2) Modem or its power brick fails from bad capacitors is not a reason for me to replace it. It is true that much of the consumer grade network gear I've bought, has failed in about 5 years of 24/7 service. I've recapped at least 3 GbE switches, and a couple routers when they failed. My Motorola/Arris modems have fared better, I had to replace a power brick on one but that's it before #1 above caused the modem to be replaced.
Since then, the last GbE switches, and routers I bought, I put better capacitors in them before even deploying them, right after making sure they weren't DOA or otherwise unsuited for my needs out of the box. Warranty be damned, I have never had a repeat capacitor failure after a repair using higher quality caps.
The annoying part is when those !@#$ AC-DC power bricks are welded shut and you have to take a bench vice and chisel (or whatever method you prefer, a C-clamp and screwdriver will work in a pinch or just a saw (lol) to get them open but if you use a saw then cementing them back together isn't nearly as easy.
Since most of those power bricks are 12V or 5V and have a standard sized DC barrel jack, I just pull one that fails, swap in a spare that I've already repaired, and put the pulled brick on the "repair this the next time you have your soldering iron out", pile. After it gets repaired, I drill holes in the casing on a drill press so they run much cooler. I don't have little people running around poking paperclips in things so that's safe enough to do. It might seem like a lot to do but frankly, the few minutes spent is less than it would take me to pick out a new widget.
If anyone wants some part #s, the two caps I buy the most of these days for several different repairs are Digikey P122393-ND and P122380-ND. Those two are 16V rated and respectively 8mm and 10mm diameter, under 18mm tall so they work in other height constrainted products like TVs and computer monitors too. It used to be that you had to aim closer to the original cap voltage to get something with enough capacitance in the same can size but Panasonic has gotten ESR so low and capacitance so high that it no longer made sense to me to buy the lower voltage values.
These two cap model #'s will work in 95% of the capacitor failures I come across, except for the older generations of video cards and motherboards which may have additional, shorter height constraints or better to go with solid/polymer caps for the higher heat resistance near a row of 'fets.